1. Field of the Invention
The present invention relates to apparatus for measuring or analyzing a video signal within a positionable window area in a video image. Such an apparatus is particularly useful for measuring a video signal produced by a medical imaging system, for example, an X-ray imaging system.
2. Description of the Related Art
In commonly-owned U.S. Pat. No. 5,537,047, a device is disclosed for measuring noise content of a video signal, rather than its video level, within a positionable window area of fixed size in a video image produced by an X-ray imaging systems
X-ray imaging systems of a type utilizing an X-ray image intensifier and camera chain to detect successive X-ray images and convert them to monochrome video images are being employed in a growing number of examinations. In addition to successive images which show the internal structure of an object under examination being available as a real time video display on a monitor, for example in fluoroscopy applications, the digitizing of these video images allows for digital image processing and/or archiving in a mass storage device. In order to set up such an imaging system and to periodically maintain or adjust its image quality, video levels (grey levels) and corresponding monitor light outputs are measured while one or more standard phantoms are employed as the objects under examination to determine whether the system's response characteristics are within standard levels. Measurement of video level may be done with an oscilloscope using a device such as described in commonly-owned U.S. Pat. No. 5,144,430, which generates a positionable marker in a video image and a corresponding oscilloscope trigger signal to allow a trace of the video line at which the marker is positioned to be displayed on the oscilloscope. Traces may be obtained and observed at various video lines corresponding to different X-ray attenuation levels in the phantom. Unfortunately, visual estimation of each pertinent video level from the oscilloscope trace is a subjective exercise which may vary between persons making the estimations and over the course of time. Further, in order to also determine the light image response of a monitor, light level is measured with a light meter at one or more predetermined places in the image of the phantom as it appears on the screen of the monitor. However, because the video level is somewhat irregular in the video supplied to the monitor by imaging a phantom, the light level measurements do not sufficiently isolate the video level to light output transfer function of the display alone.